1. Field of the Invention
The present invention relates to use of a transmitter in a communication system. More particularly, the present invention relates to calibration of a transmitter.
2. Description of the Related Art
In the case of a transmitter/receiver using Frequency Division Duplexing (FDD), noise generated in a Radio Frequency (RF) circuit of the transmitter has an effect on transmission/reception performance. Among the noise generated in the transmitter, an out-of-band noise, more particularly, a noise located in a receive band, increases a noise floor of the receiver, which results in deterioration of reception sensitivity. At present, by inserting an external fixed frequency Surface Acoustic Wave (SAW) filter between the RF circuit and a Power Amplifier Module (PAM) in order to remove the noise having an effect on the receiver, the transmitter RF circuit used in a portable terminal suppresses the increase of the noise floor by the use of the SAW filter, and allows a unique noise characteristic of the PAM to be maintained. The RF circuit can be called an RF Integrated Circuit (RFIC). In this case, the noise floor of the PAM is preferably designed to be low enough to prevent deterioration of the reception sensitivity of the receiver.
Systems of various standards and frequencies have recently been developed for mobile communication systems. In addition, various efforts have been made to include the systems of various standards and frequencies in one terminal by using a recently developed transmitter RF circuit. In case of using a terminal that supports a multi-mode and a multi-band, when the conventional transmitter structure is used, it is required to have a plurality of external, fixed frequency SAW filters according to the different frequencies to be supported. This causes an increase in the overall system size and an increase in the manufacturing cost. Therefore, a technique for removing the external, fixed frequency SAW filters while supporting the multi-mode and the multi-band has recently emerged as an issue in the manufacturing of the terminal.
However, since the transmitter RF circuit takes a role as a signal source, it must be implemented according to a correct requirement standard. If the SAW filters located between the RF circuit and the PAM are removed without any supplemental measures, an out-of-band noise of the transmitter RF circuit is directly delivered to the PAM. As a result, the noise is amplified due to an out-of-band gain of the PAM, and an output of the PAM may have significant noise. A magnitude of the amplified noise of the transmitter RF circuit significantly surpasses a magnitude of a unique output noise of the PAM, and thus reception sensitivity deterioration caused by the rising of the noise floor of the receiver still remains as a problem.
As described above, although there is a desire to decrease a device size and a manufacturing cost by removing the SAW filters, the removal of the SAW filters causes a problem in that the out-of-band noise of the transmitter is significantly increased. Accordingly, there is a need for a method for effectively removing the out-of-band noise.